Is there a way to non-lethally take down the heavy enemies wearing exosuits without being detected?

Shoot them with an EMP round and then do a non-lethal takedown?

Thanks, now I feel really dumb! I haven't even used any weapons other than the tranq rifle up to this point.

Word of warning to other pacifists: Non-lethal takedown can sometimes kill the target.

I noticed this last night after knocking out a non-combatant (technician) during early story missions. For some reason, takedown was lethal for that NPC but fortunately stun-gun and tranq-dark worked as intended, so make sure to check the status of each person you've knocked out. I've probably ruined my pacifist-run at this point because I noticed this by the time I was already 10h into the game. I tested this later on civilians on the streets but they survived the same move just fine, so I'm not sure what's going on. Maybe it's something to do with positioning, for example getting killed by physics after released from the animation (like getting crushed against environment).

I've noticed that if the animation gets out-of-sync with the target's character model (eg Jensen punches the floor or wall instead of the target), the target will probably be killed by clipping into geometry.

Stun gun then non lethal takedown

@dharmabum: I know this has been answered already, but yeah, EMP shot + non-lethal takedown works. Animation looks brutal too: